This invention relates to methods for controlling electro-optic displays. In one aspect this invention relates to providing a reduced power state in an electro-optic display, and more specifically to an active matrix electro-optic display using a bistable electro-optic medium, the display being provided with means for controlling the potential at a common electrode during a non-writing state of the display. In another aspect, this invention relates to methods for controlling electrode voltage in electro-optic displays, and more specifically to methods for controlling the voltage applied to the common front electrode of an active matrix electro-optic display using a bistable electro-optic medium.
Electro-optic displays comprise a layer of electro-optic material, a term which is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the imaging art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. No. 6,301,038, International Application Publication No. WO 01/27690, and in U.S. Patent Application 2003/0214695. This type of medium is also typically bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,750,473; and 6,753,999; and U.S. Patent Applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; 2003/0053189; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; and 2004/0112750; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/092077; WO 03/107315; and WO 2004/049045.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned 2002/0131147. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Certain of the aforementioned E Ink and MIT patents and applications describe electrophoretic media which have more than two types of electrophoretic particles within a single capsule. For present purposes, such multi-particle media are regarded as a sub-class of dual particle media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within capsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and U.S. Patent Application Publication No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
To obtain a high-resolution electro-optic display, individual pixels of the display must be capable of being addressed without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, which may be transistors or diodes, with at least one non-linear element being associated with each pixel of the display. A pixel or addressing electrode adjacent the relevant pixel is connected via the non-linear element to drive circuitry used to control the operation of the display. Displays provided with such non-linear elements are known as “active matrix” displays.
Typically, such active matrix displays use a two-dimensional (“XY”) addressing scheme with a plurality of data lines and a plurality of select lines, each pixel being defined uniquely by the intersection of one data line and one select line. One row (it is here assumed that the select lines define the rows of the matrix and the data lines define the columns, but obviously this is arbitrary, and the assignments could be reversed if desired) of pixels is selected by applying a voltage to a specific select line, and the voltages on the data or column lines are adjusted to provide the desired optical response from the pixels in the selected row. The pixel electrodes in the selected row are thus raised to voltages which is close to but (for reasons explained below) not exactly equal to the voltages on their associated data lines. The next row of pixels is then selected by applying a voltage to the next select line, so that the entire display is written on a row-by-row basis.
When the non-linear elements are transistors (typically thin film transistors (TFT's)), it is conventional practice to place the data and select lines, and the transistors, on one side of the electro-optic medium, and to place a single common electrode, which extends across numerous pixels, and typically the whole display, on the opposed side of the electro-optic medium. See, for example, the aforementioned WO 00/67327, which describes such a structure in which data lines are connected to the source electrodes of an array of TFT's, pixel electrodes are connected to the drain electrodes of the TFT's, select lines are connected to the gate electrodes of the TFT's, and a single common electrode is provided on the opposed side of the electro-optic medium. The common electrode is normally provided on the viewing surface of the display (i.e., the surface of the display which is seen by an observer). During writing of the display, the common electrode is held at a fixed voltage, known as the “common electrode voltage” or “common plane voltage” and usually abbreviated “VCOM”. This common plane voltage may have any convenient value, since it is only the differences between the common plane voltage and the voltages applied to the various pixel electrodes which affects the optical states of the various pixels of the electro-optic medium. Most types of electro-optic media are sensitive to the polarity as well as the magnitude of the applied field, and thus is necessary to be able to drive the pixel electrodes at voltages both above and below the common plane voltage. For example, the common plane voltage could be 0, with the pixel electrodes varying from −V to +V, where V is any arbitrary maximum voltage. Alternatively, it is common practice to hold the common plane voltage at +V/2 and have the pixel electrodes vary from 0 to +V.
One important application of bistable electro-optic media is in portable electronic devices, such as personal digital assistants (PDA's) and cellular telephones, where battery life is an important consideration, and thus it is desirable to reduce the power consumption of the display as far as possible. Liquid crystal displays are not bistable, and hence an image written on such a display must be constantly refreshed if the image is to remain visible. The power consumed during such constant refreshment of an image is a major drain on the battery. In contrast, a bistable electro-optic display need only be written once, and thereafter the bistable medium will maintain the image for a substantial period without any refreshing, thus greatly reducing the power consumption of the display. For example, particle-based electrophoretic displays have been demonstrated in which an image persists for hours, or even days.
Thus, it is advantageous to stop scanning an active matrix bistable electro-optic display between image updates to save power. In some cases even more power can be saved by fully powering down the drivers and common plane circuits used to drive the display.
However, implementation of the necessary non-writing mode (alternatively referred to as the “non-scanning” or “zero power” mode) is not trivial. The display should be designed and operated in such a manner that no significant voltage amplitude transients are experienced by the electro-optic medium as the display switches between its writing (scanning) mode and its non-writing modes.
At first glance, it might appear that simply loading the column drivers with the midpoint voltage (i.e., the voltage which is the mid-point of the range used by these drivers), and stopping the gate driver clock with no gate lines selected would be an acceptable way to implement the non-writing mode. However, in practice this would lead to a steady state DC bias current being applied to the electro-optic medium. Any active matrix display suffers from an effect called “gate feedthrough” or “kickback”, in which the voltage that reaches a pixel electrode is shifted by some amount (usually 0.5-2.0V) from the corresponding column (data) voltage input. This gate feedthrough effect arises from the scanning of the gate (select) lines acting through the coupled electrical network between gate lines and source lines/pixel electrodes. Thus, the voltages actually applied to the pixel electrodes are shifted negatively from the column driver voltages because of the gate feedthrough during scanning. Normally, the common plane voltage is offset negatively from its notional value by a fixed amount to allow for this gate feedthrough shift in the voltages applied to the pixel electrodes. When scanning is stopped, this shift due to gate feedthrough will not occur and the column driver mid-point voltage will then be higher than that required to generate zero voltage difference between the common plane and pixel electrodes. The TFT's will accordingly leak current between the column lines and the pixel electrodes under this bias according to their off state characteristics, and this current will flow from the pixel electrodes through the electro-optic medium to the common electrode. This current flow will in turn generate a voltage across the electro-optic medium, and this voltage is undesirable because such it can disturb the optical state of the electro-optic medium during the non-writing period and can also lead to reduced material lifetime and the buildup of charges in the electro-optic medium that will adversely affect the optical states of subsequent images after scanning is resumed. (It has been shown that at least some electro-optic media are adversely affected if the current therethrough is not DC balanced over the long term, and that such DC imbalance may lead to reduced working lifetime and other undesirable effects.)
Furthermore, although at first glance it might appear that powering down the driver circuitry in preparation for a non-writing mode only requires that the circuitry supplying biasing voltages be shut down, or that the flow of power from such circuitry to the drivers be interrupted, in practice either measure is likely to provide undesirable voltage transients to the electro-optic medium; such voltage transients may be caused by, inter alia, parasitic capacitances present in conventional active matrix driver circuitry.
In one aspect, the present invention seeks to provide apparatus for, and methods, of implementing, a non-writing mode in an electro-optic display without imposing undesirable voltage transients on the electro-optic medium during switching of the display into and out of the non-writing mode. The present invention also seeks to provide apparatus for, and methods, of implementing a non-writing mode in an electro-optic display without undesirable voltage offsets on the electro-optic medium that could adversely affect this medium.
Other aspects of the present invention relate to methods for measuring and correcting voltage offsets. The origin of gate feedthrough voltage has been explained above. Ideally, the gate feedthrough voltage is roughly equal across all the pixels in an array and can be cancelled out by applying an offset to the common electrode voltage. However, it is difficult to apply to the common electrode an offset voltage that almost exactly cancels out the feedthrough voltage. In order to do so, means must be provided to ascertain whether the offset voltage accurately matches the feedthrough voltage, and to generate, set and adjust the offset voltage. Ideally, the feedthrough voltage would be known beforehand and the offset voltage could be set permanently and cheaply at the time the display electronics are manufactured. In practice, some adjustment of offset voltage is required after the electronics and the display are assembled as a final unit.
In conventional liquid crystal displays (LCD's), adjustment of the offset voltage can be effected by eye; when an incorrect offset voltage is applied, the eye will detect a flickering of the display. The offset voltage can then by adjusted by an operator varying an analog potentiometer until the flicker disappears.
However, in particle-based electrophoretic displays, and in most other types of bistable electro-optic displays, an incorrect offset voltage will not cause any effects visible to the human eye unless the error in the offset voltage is very large. Thus, substantial errors in offset voltage can persist without being observable visually, and these substantial errors can have deleterious effects on the display if left uncorrected. Accordingly, it is highly desirable to provide some method other than visual observation to detect errors in the offset voltage. Furthermore, although such errors, once detected and measured, can be corrected manually in the same way as in LCD's, such manual correction is inconvenient and it is desirable to provide some way of adjusting the offset voltage automatically.
The present invention seeks to provide apparatus for, and methods of, measuring and correcting offset voltage. The present invention extends to both manual and automatic correction methods.